1. Field of the Invention
The invention relates to a method and arrangement for improving the accuracy of time measurements related to positioning in a radio system.
2. Description of the Related Art
Interest in and need for positioning subscriber terminals of cellular networks, i.e. defining the geographical location of subscriber terminals, has lately increased significantly. By utilizing positioning, it is possible to implement numerous commercial services, such as navigation services assisting a user, fleet (trucks, taxis, busses) monitoring and management services, or services for the positioning of children or other relatives and friends. Network operators can utilize positioning in defining different tariff zones or for the provision of targeted advertisement services. The authorities are also interested in positioning. For instance, the Federal Communication Commission of the United States has requested that it be possible to position all subscriber terminals making an emergency call at an accuracy of 50 meters. Recommendations for the positioning of subscriber terminals making emergency calls also exist in the European Union.
A positioning service can be implemented in several ways. On the most basic level, a subscriber terminal can be positioned on the basis of the identity of the cell serving it (cell ID based method). The obtained result is, however, not very accurate, because one cell can cover dozens of kilometers.
A better result is obtained by using as additional information timing information of the radio link, such as the timing advance (TA). In the GSM (Global System for Mobile Communications) system, for instance, TA indicates the location of a subscriber terminal at an accuracy of approximately 550 meters. However, the positioning accuracy varies depending on the used antenna solution. If the cell has an omnidirectional antenna, for instance, the location of the subscriber terminal is known relative to a base station on a circle drawn around it. Sectoring the base station into three parts, for instance, improves the situation somewhat, but even then the subscriber terminal can only be positioned to a 120-degree sector in a 550-meter deep area at a specific distance from the base station.
Even the above inaccurate methods are sufficient for some applications, such as for defining tariff zones. In addition, other more accurate methods have, however, also been developed.
These methods include uplink methods that are based on the fact several different base stations perform measurements on the signal transmitted by the subscriber terminal. One example of these is the TOA (Time of Arrival) method.
One positioning method is also the use of a GPS (Global Positioning System) receiver located in the subscriber terminal. The GPS receiver receives a signal transmitted by at least four earth-orbiting satellites, on the basis of which it is possible to calculate the latitude, longitude and altitude of the location of the subscriber terminal. The subscriber terminal can perform the calculation independently or it can be assisted, in which case it is called network-assisted GPS positioning.
In downlink methods, the subscriber terminal makes measurements on signals transmitted by several different base stations. One example of such a method is the E-OTD (Enhanced Observed Time Difference) method. Because the radio network is never fully synchronous in practice, the actual timing of the signals transmitted by the base stations must be measured. This can be done for instance by using a location measurement unit (LMU) located at a fixed, known measuring point. The effect of the actual time differences between the transmissions of defined base stations is removed by using the location measurement unit from the results measured by the subscriber terminal, after which the subscriber terminal can be positioned geometrically on the basis of the coordinates of the base stations, for instance to the intersection of hyperboles or circles depicting the propagation delay.
The 3GPP (3rd Generation Partnership Project) specifications define as the positioning methods supported by the radio access network of UMTS (Universal Mobile Telecommunications System), which represents the third-generation systems, not only the cell ID based method and the network-assisted GPS method, but also the OTDOA (Observed Time Difference of Arrival) method and its variant OTDOA-IPDL (Idle Period Downlink).
The OTDOA method can be considered a 3G-system counterpart for the E-OTD method. The OTDOA-IPDL method also utilizes the time instants when the base station cuts its transmission for a short time. During this time instant, the terminals of the cell can measure other base stations, and RTD measurements can be made.
The prior-art positioning methods are thus generally based on measuring the signals of the base stations and the timing differences between base stations. The biggest problem with the positioning accuracy is thus the accuracy of the time measurements related to positioning.
The time difference between base stations can be defined using their real time differences (RTD) that can be defined using a location measurement unit, for instance, on the basis of the signals the unit receives from the base stations. Different positioning methods, such as the E-OTD method, can also be applied by using what is known as the absolute time (AT) that can be defined relative to the GPS time defined using a GPS receiver. The GPS receiver can be located in the location measurement unit, for instance. Attempts have been made to reduce errors in time definition by improving the accuracy of the GPS measurement, for instance, but this is still a problem.
The receivers used in the measurements performed in positioning comprise, depending on the used architecture, various filters, amplifiers and DSP (Digital Signal Processor) structures, which all cause a propagation delay of their own to the receiver. These form together the group propagation delay of the receiver. The propagation delays in the different parts of the receiver vary depending on the variation of temperature, input power and supply voltage, as well as on unit-specific variations. Each of the above-mentioned variation may cause a variation of approximately 1 ns to 1 μs in each part of the receiver. In the receiver used for positioning, the variations may cause propagation delay variations of such magnitude that the positioning accuracies defined in the specifications cannot be reached.
Attempts have been made to compensate the variation of the propagation delay by using in the different stages of the receiver high-quality components, with which a certain controllable delay has been reached. The problem with this solution is the extra costs that arise from the use of the better quality components.